Diaryl carbonates such as diphenyl carbonate may be prepared by a process wherein a dialkyl carbonate such as diethyl carbonate or dimethyl carbonate is reacted with an aryl alcohol such as phenol. The process is often carried out in two steps: a first step wherein the diaryl carbonate is reacted with the aryl alcohol in the presence of a transesterification catalyst to provide an alkyl aryl carbonate, and a second step wherein the alkyl aryl carbonate undergoes disproportionation to provide a diaryl carbonate and a dialkyl carbonate. Examples of such processes are disclosed in U.S. Pat. No. 5,344,954, WO2011014374 and WO2011067263.
The process is typically carried out in a series of distillation columns, some of which are operated at subatmospheric pressure. The present inventors have observed that air can leak into the distillation columns operated at subatmospheric pressure with a number of negative consequences. Oxygen and moisture from the air can cause side reactions to form undesirable trace impurities and colour bodies. They can also react with the catalyst, causing it to deactivate and, with homogeneous catalysts, precipitate out of the reaction solution. The present inventors have sought to provide a process which avoids these problems.